The present invention relates generally to implants introduced into the body, particularly to fill material for such implants, and specifically to rheologically modified fill material for implants.
For implants such as breast and testes prosthesis as well as other implants and prosthesis, silicone has been the fill material of choice. However, silicone as a fill material has fallen into disfavor. This has prompted efforts to find replacements for silicone. These replacements are often if not always undesirable because such replacements have been unable to match the feel provided by silicone. The inventors of the present invention have investigated the rheological parameters of certain fluid formulations in an effort to provide a fill material having the heretofore unmatched feel of silicone. Thus, it is important to understand some basics of rheology to have an understanding of the present invention.
Rheology is the science of the deformation and flow of matter. It is concerned with the response of materials to mechanical force. Polymer rheology deals with polymeric materials and biorheology deals with biological fluids.
Deformation is the relative displacement of points of a body and can be divided into two general types: flow and elasticity. Flow is irreversible deformation; when the stress is removed, the material does not revert to its original configuration. Elasticity is reversible deformation; the deformed body recovers its original shape.
The usual way of defining the rheological properties of a material is to determine the resistance to deformation. Resistance to deformation is measured by two indexes or yardsticks: 1) viscosity (the index or yardstick for flow; viscosity is the resistance to flow of a liquid); and 2) the degree of elasticity (elastic deformation).
A liquid is a material that continues to deform as long as it is subjected to a tensile or shear stress. For a liquid under shear, the rate of deformation (shear rate) is proportional to the shearing stress.
Thixotropy is the decrease in viscosity with time when sheared at a constant shear rate. Rheopexy, a relatively rare occurrence, is the increase in viscosity of a fluid in response to shear. For example, as to thixotropy, when a shearing action begins, such as when one applies a latex house paint with a brush, the viscosity decreases quickly to permit the paint to be easily brushed to a thin film and provide a short period of time for the brushmarks to level. When the shearing action stops, such as when the paint leaves the brush and clings to the wall, the viscosity of the latex house paint increases to prevent running and sagging. Thixotropy may be a time dependent effect.
A single fluid may be subject to a number of shear rates. For example, a paint may be pumped during manufacture or immediately prior to application (intermediate shear rate), sprayed onto a wall (high shear rate), coalesce and flow to form a uniform film (intermediate to low shear rate), and sag or run under gravity (low shear rate). A given liquid or material may work well at one or two of the shear rates, but fail at other shear rates.